²⁷ Al NMR diffusometry of Al₁₃ Keggin nanoclusters

Area of Science:

• Geochemistry
• Materials Science
• Analytical Chemistry

Background:

• Aluminum hydroxide clusters, such as ϵ-Al13, are crucial in aluminum speciation and mineral transformations.
• Direct measurement of their translational diffusion is challenging and often relies on indirect methods.

Purpose of the Study:

• To apply 27Al pulsed field gradient stimulated echo nuclear magnetic resonance (PFGSTE NMR) spectroscopy for direct measurement of ϵ-Al13 cluster diffusion.
• To validate experimental findings with theoretical simulations and existing literature data.

Main Methods:

• Utilized 27Al PFGSTE NMR spectroscopy to probe the AlO4 core of the ϵ-Al13 cluster.
• Performed complementary theoretical simulations to calculate diffusion coefficients and hydrodynamic radii using boundary element methods.
• Applied the Stokes-Einstein relationship to correlate diffusion coefficients with hydrodynamic radii.

Main Results:

• Successfully measured the translational diffusion of the ϵ-Al13 cluster using 27Al PFGSTE NMR.
• Experimental diffusion coefficients and hydrodynamic radii showed good agreement with theoretical simulations and literature data.
• Demonstrated that the symmetric AlO4 core of ϵ-Al13 has favorable NMR relaxation properties for PFGSTE NMR studies.

Conclusions:

• 27Al PFGSTE NMR diffusometry is a viable technique for probing symmetric aluminum environments in polynuclear clusters.
• This method expands the applicability of NMR diffusometry to larger molecular weight clusters than previously studied.